The present invention relates to generally a suspension for two-wheeled vehicles and more particularly a front fork or rear damper or suspension for suspending a front or rear wheel of a motorcycle.
In general, a front fork or rear damper of a motorcycle comprises telescopic shock absorbers and suspension springs. The front or rear wheel of the motorcycle is suspended by the suspension springs, and the oscillations of the suspended springs are absorbed by the telescopic shock absorbers. The road shock caused by the passage of the wheels over irregularities is absorbed by the front fork and the rear damper within a relatively shorter time. In this case, the frequency of oscillation is dependent upon the load applied to the suspension springs and also upon the spring constants of the suspension springs, so that the oscillation of the motorcycle with the front fork and rear damper to which are applied the heavy loads or which have the soft suspension springs is shorter than that of the motorcycle with the front fork and rear damper to which are applied the light loads or which have the stiff suspension springs.
In all of the front forks and rear dampers of the type described, the compression and expansion strokes of the suspension springs are limited to a predetermined distance from the standpoint of design of a motorcycle. It is therefore required that the suspension springs must deflect or compress within a predetermined compression stroke when the load in a predetermined range is applied. In view of the above, the suspension springs used in the front forks and rear dampers must have as smallest a spring constant as possible.
The rate of deflection of a helical spring is generally determined uniquely depending upon its spring constant, and remains unchanged in the whole stroke. Therefore, if the coil spring with a small spring constant is used in order to attain the desired spring characteristics within a predetermined load range, the riding quality is improved but the helical spring tends most often to be most compressed and most expanded. As a result the suspension springs are over-compressed and readily reach the limits of the stroke, over-expanded, thus resulting in the poor riding quality. To overcome this problem there have been proposed various methods which generally provide the non-linear spring characteristics. In one method, a coil spring has different diameters and pitches of coils throughout its whole length. In another method, a plurality of helical springs are disposed in parallel or in series so that the spring constant as a whole may be varied depending upon the load. However, even if it is possible to raise steeply the load-deflection curve when the suspension spring is compressed it is impossible to raise the curve when the suspension spring is expanded so that the problem of over-expansion cannot be overcomed.
Furthermore, the ratio of the weight of a driver and/or passenger to that of the motorcycle is extremely larger compared with the four-wheeled vehicles or the like so that in order to provide the satisfactory displacement of the load, the helical springs of the front forks and rear dampers must have a high spring constant. This results in the undesired high frequency of oscillation when the load is light. To overcome this problem, there has been devised and demonstrated a suspension system in which a present load may be externally adjustable depending upon the loads applied to the front fork and rear dampers so that the spring constant of the helical spring may be maintained at relatively small magnitude under all load conditions. Even though the better riding quality may be maintained in the wide range of loads by changing the initial loads applied to the coil springs, the spring constant remains unchanged. Therefore, when the load is heavy so that the initial load applied to the helical springs are increased, the reaction of the springs is increased but the over-expansion of the springs cannot be prevented, thus bringing about the problems of impact and noise.